1. Field of the Invention
The present invention relates to a displacement measuring method and apparatus, particularly, to a displacement measuring method for performing non-contact measurement of surface positions and surface displacements of measurement objects.
2. Description of the Related Art
FIG. 1 shows a configuration of a conventional displacement measuring apparatus 1. The displacement measuring apparatus 1 uses a confocal optical system in which optical beams emitted from a light source 2, such as a laser diode, are passed through a collimating lens 3, are converted to parallel optical beams, and are passed through a beam-splitter cube 4. After passing through the beam-splitter cube 4, the optical beams are converged by an objective lens 5, and the converged optical beams are incident on a surface of a measurement object 6. The optical beams, having been reflected off the surface of the measurement object 6, are passed through the objective lens 5 again, are incident on the beam-splitter cube 4, and are reflected by 90 degrees therethrough. The optical beams having been reflected by 90 degrees through the beam-well-balanced stable optical-intensity curve, as shown in FIG. 2. This allows the position to be measured with high precision.
When the surface of the measurement object 6 has irregularities, however, optical-intensity curves shown in FIGS. 3 and 4 are produced. Determination of the point z.sub.0 which is equivalent to the optical-intensity maximum (the position of the objective lens 5 at the optical intensity maximum value) is difficult, preventing the position and variations to be measured with high precision.
Also, with a method that detects the position of a surface of a measurement object according to the maximum value of an optical-intensity curve, when a high optical intensity is suddenly detected because of noise and the like (as shown in FIG. 10), erroneous measurement results are produced. This causes measurement operation to be inconsistent.
Referring back to the above, the lens surface is a surface whose roughness Ra is at most 0.01 .mu.m. A surface having irregularities means a surface whose roughness ranges from 0.1 .mu.m to 1 .mu.m.